Change it to Sheetdonkey. Donkeys are cool and you only have to change one letter everywhere.

As someone who had to research this recently, but is not a lawyer (so take this with a grain of salt): I would find it highly unlikely that a legal challenge arises over this. You are not doing anything on your site to mislead customers or cause confusion with surveymonkey, you clearly did not act in bad faith, it would be impossible to collect evidence that actual customer confusion occurred, and it is reasonable to think that customers looking to send surveys online are sophisticated enough to determine the products apart.

The only possible trademark issue I can think of (I can't remember the exact case here, but it had to do with Polaroid) - a trademark holder can claim they intend to expand their operations to use the name, so SurveyMonkey could theoretically say they plan on expanding to [xyz]monkey without any obligation to actually do it.

I'd change the name now while it's still early, so people won't have to go hunting for the new name later.

This is the last thing you should be worried about

They didn’t go after Mailchimp. A good sign.

Really unfortunate, love the artwork!